Mark Duplass, left, and Joshua Leonard in a scene from the film 'Humpday,' which the New Orleans Film Society will screen Monday night (6/7).

Nothing could prepare New Orleans native Mark Duplass -- half of the blissfully in-demand Duplass brothers directing team -- for his big moment in the final days of filming the indie comedy "Humpday."

Duplass usually co-directs, but this role is one of the occasional solo projects he undertakes without brother Jay. The big moment came when he and four others checked into a cramped, anonymous hotel room for the film's final payoff scene. Since the movie's dialogue was entirely improvised, there was no script. If there had been one, it would have said something like this:

Mark and Josh undress, then they kiss.

"It was unnerving," Duplass said, laughing while discussing the film, which the New Orleans Film Society will screen Monday night at 7:30 at the Canal Place Cinema.

Directed by Lynn Shelton, the low-budget "Humpday" is about two old friends -- played by Duplass and his co-star and real-life friend Joshua Leonard -- who reconnect after living separate lives for the better part of a decade. Partly out of bravado, partly out of drunkenness, they decide to participate in an alternative film festival seeking to "reclaim" pornography for the art world.

Their booze- and pot-fueled idea: two straight men having gay sex. Once they sober up and turn the camera on, however, things get a little tense. But they keep going. For reasons of their own, neither character wants to be the one to back out.

Mark Duplass, left, and Joshua Leonard in 'Humpday.'

'HUMPDAY' SCREENING

What: The New Orleans Film Society hosts a screening of the indie comedy, starring local native Mark Duplass.

When: Monday (July 7), 7:30 p.m.

Where: Canal Place Cinema, 333 Canal Street, third floor

Tickets: $8 general admission, $6 for Film Society members. Available at Canal Place Cinema box office.

In keeping with the aesthetic of the "low-fi" mumblecore genre -- in which Shelton and the Duplasses have become fixtures -- the film was shot on a shoestring and built around improvised dialogue.

"We didn't make any structure for the last scene," Duplass said, "and we decided we weren't going to talk about it. We were just going to check into a hotel room at seven o'clock at night and check out at seven o'clock in the morning, and see what we got."

As a result, nobody -- not the actors, not the director, not the crew -- really knew how the movie would end.

"He's a strong kisser," Duplass said of Leonard's approach to their awkward, aggressive on-screen smooch. "Dudes' mouths are bigger, or at least Josh's mouth was bigger, than the average female I've dealt with. ... It was like a handshake. There's a lot of strength in it."

As awkward as it was for the happily married Duplass, the experience also was a blast.

"It's kind of frightening how fun it is to just act in a movie, and particularly a movie like 'Humpday,' because there was no actual written dialogue for the movie. There was a detailed scene outline, but we improvised the whole thing. ... You are acting, but a little bit of you is writing the scene as you go. Ten percent of your brain is thinking about, 'How am I going to nail getting from point B to point C?' 'Oh, he just threw that at me, that'll bring me to D.'

"I really like that aspect of acting a lot, because it involves my writer brain as well."

It's a format he has worked in his whole career, but "Humpday" still was a departure for Duplass, mostly because he was working without his brother and creative partner, Jay.

"That being said, and Jay can attest to this, too -- and sometimes Jay goes off and shoots short documentary subjects and things that he's interested in exploring, and I go off and do my acting projects -- when you're that intrinsically linked with someone creatively, like Jay and I are, it is nice every now and again to break out and feel like you can do something on your own."

It's something Duplass is getting more and more chances to experience. In addition to "Humpday," he has a major role in the indie comedy "True Adolescents," as well as in director Noah Baumbach's 2010 dramatic comedy "Greenberg," in which he appears with Ben Stiller.

"There's nothing like the communication that Jay and I have, and there never will be anything like that," Duplass said. "When Jay and I are making a movie and something is happening and it's right, we don't even have to look at each other. It's like I feel it and he feels it -- it's like 'The Corsican Brothers.' I don't have that shorthand with anyone else."